My favorite filter in Pic!

Here is the filter that kills greens and blues but leaves flesh tones alone.

On the left is the original and right is Pic! output.   It's obvious the sky goes from blue to grey but less obvious is the trees on the right are brown (in RGB G is never greater than R).    The trees may still look green but I think that is due to context and contrast.  However it may just be that green is a powerful color: my read of the xkcd data (read that if you haven't it's fantastic!) is that it would be "olive" for most people.